The record of the past half century has established, I think, the two general principles about human disease. First, it is necessary to know a great deal about underlying mechanisms before one can really act effectively; one had to know that the pneumococcus (肺炎球菌) causes lobar pneumonia (球形肺炎) before one could begin thinking about antibiotics. Second, for every disease there is a single key mechanism that dominates all the others. If one can find it, and think ones way around it, one can control the disorder. This generalization is harder to prove and arguable—it is more like a strong hunch (直觉) than a scientific assertion— but I believe that the record thus far tends to support it. The most complicated, multicell, multi-tissue, and multiorgan diseases I know are tertiary syphilis, chronic tuberculosis, and pernicious anemia. In each, there are at least five major organs and tissues involved, and each appears to be affected by a variety of environmental influences. Before they came under scientific appraisal, each was what we now call a 'multifactorial disease. ' And yet, When all the necessary facts were in it was clear that by simply switching off one thing—the spirochete, the tubercle bacillus, or a single vitamin deficiency—the whole array of disordered and seemingly unrelated pathological mechanisms could be switched off, at once. What is the main idea of the passage?